1. Field of the Invention
The invention relates to an integrated data driver structure used in a current-driving display device, and more particularly, to a current-storing/reproducing integrated data driver structure including a digital-to-analog current converter and a plurality of grades of data drivers.
2. Description of the Prior Art
An OLED is an electrically driven lighting element having a brightness that depends on the magnitude of a related current. At present, the magnitude of the brightness (which is also called the gray-scale value) is controlled by the magnitude of the OLED driving current in an application OLED matrix display. Base upon the driving method, the matrix display can be classified as either a passive matrix or an active matrix display. Passive matrix displays adopt the method of driving the scan lines of the display in sequence, driving pixels in different rows sequentially. Since the light-emitting time of each pixel is restricted by the scanning frequency and the numbers of scan lines, the passive matrix method is not suitable for large-sized and high dots-per-inch (dpi) displays. Active matrix displays, however, possess an independent pixel circuit for each pixel, which is described in FIG. 1, which is a schematic diagram of a pixel 20. The present embodiment of the pixel 20 includes a capacitor C1, an OLED D, and a plurality of MOS transistors or TFTs (Thin-film Transistors) T1–T4. With this arrangement, even in large-sized and high dpi displays, a steady driving current I is provided for each pixel, which improves the brightness balance.
For achieving advantages of power saving, integrity, and cost effectiveness, more OLED systems adopt the digital type as an input data type so that the digital-to-analog converter should be involved in the data driver. In addition, the brightness of the OLED display is controlled by current. Therefore, the digital-to-analog process should be achieved by a digital-to-analog current converting circuit to convert digital data into an analog current signal. The corresponding pixel is also a current-driving pixel as the pixel 20 as shown in FIG. 1. Please refer to FIG. 2, which is a functional block diagram of a prior-art data driver structure 11. The data driver structure 11 corresponds to a plurality of matrix-arranged pixels 20 of a display device, and each of the pixels 20 is shown in FIG. 1. A plurality of scan lines 21 corresponding to the plurality of pixels 20 are included in the embodiment shown in FIG. 2. The data driver structure 11 includes a shift register 16 and a plurality of grades of data drivers 10. Each grade of data driver 10 includes a level shifter 12, a latch 14, and a digital-to-analog current converter 18. In a period of time there is at least a grade of data driver being operated to receive a digital signal. The level shifter 12 of the grade of data driver 10 adjusts potential levels of the digital signal (6-bit digital signal). The latch 14 is electrically connected to the level shifter for executing level-shifting and buffering functions. The latch 14 can be used to latch the 6-bit digital signal. The shift register 16 can be used to generate a shift-register signal to transmit the digital signal to the level shifter 12 at one time. Afterwards, the digital signal will be transmitted to the latch 14.
Please continue to refer to FIG. 2, the digital-to-analog current converter 18 of the operated grade of data driver 10 can be used to receive the digital signal and to transform the digital data into an analog current signal. The analog current signal will then be applied to a corresponding data line 19. The data line 19 is coupled to a plurality of pixels 20. In the meanwhile, there is a scan line 21 being operated (at a high potential level) to turn on the pixels 20 connected to the scan line 21. Therefore, the digital-to-analog current converter 18 will be connected to the data line 19 and the corresponding pixel 20, and the digital-to-analog current converter 18 will be used to conduct the analog current signal to the corresponding pixel 20. The grade of data driver 10 can be used to control the gray-scale quality of the display panel according to the magnitude of the analog current signal.
J. Kanicki et.al. (U. of Michigan, USA) has disclosed a simple digital-to-analog current converter installed with a set of TFTs (Thin Film Transistors) with a width-to-length ratio assigned as 1:2:4:8 and a current source to generate 16 current gray scales. Please refer to FIG. 3, which is a schematic diagram of an embodiment of a prior-art digital-to-analog current converter 18. The digital-to-analog current converter 18 is composed of a plurality of transistors T5–T9. Due to that the 16 current gray scales rely on 4 (1:2:4:8) TFTs T6–T9, any fluctuation of threshold potential level and mobility in each TFT will generate significant variation to affect the current gray scales. Furthermore, the quality of the corresponding panel will be influenced. In addition, because the output impedance of the digital-to-analog current converter 18 is not high enough, the output potential level will be affected by a current flow passing the digital-to-analog current converter 18. Therefore, when the digital-to-analog current converter 18 is connected to the corresponding pixel, the output current may not be a stable 16 gray-scale current.